The sudden, and apparently life-threatening, illness of Omar Abdel-Rahman, the so-called blind sheik, who is serving a life sentence in federal prison, has sparked fears that that his followers might launch terror attacks against the United States should he die in custody.

The sheik was rushed to a prison infirmary last week after spitting up blood and is reportedly in grave condition.

The terror alert was sparked, not by specific information about a plot, but because the sheik has told his followers to seek revenge if he should die while in American hands.

The FBI said that the bulletin it sent to law enforcement authorities this week was merely a "prudent action," and that there is no credible evidence his fervent followers were about to stage an attack.

Law enforcement sources say that doctors discovered a large tumor on his liver and that his condition is deteriorating.

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Rahman, who was convicted based on investigations growing out of the first terror attack on the World Trade Center in 1993, was the leader of one of the most violent terror groups in mordern times.

His Egypt-based group, Al-Gama al Islamiyya, or the Islamic Group, has been responsible for numerous terror acts. The most notable was the November 1997 attack on tourists in Luxor, Egypt that laft 58 dead.

Rahman was also closely tied to Osama bin Laden. The two met in Afghanistan after Rahman was expelled from Egypt following charges he was involved in the slaying of Egyptian President Anwar Sadat.

He came to the U.S. in 1990 on a tourist visa despite his name appearing on a terrorist watch list. The sheik quickly built a following among radical Muslims in New York and New Jersey.

After the first bombing of the World Trade Center, FBI agents focused on the blind sheik and managed to slip an informer into his organization. The hidden tapes and video recordings unveiled a conspiracy to wreak havoc on the New York area in the name of radical Islam.

Federal authorities charged that Rahman and his followers were planning a coordinated attack on the Holland and Lincoln tunnels, the George Washington Bridge, the United Nations and the Manhattan Federal Building. He was arrested for "seditious conspiracy’ in October 1995 and sentenced to life in prison.

His arrest and imprisonment has been a rallying cry for jihadists around the world.